[Python-Dev] PEP 549: Instance Properties (aka: module properties)
larry at hastings.org
Sun Sep 10 23:52:12 EDT 2017
On 09/06/2017 02:13 PM, Ronald Oussoren wrote:
> To be honest this sounds like a fairly crude hack. Updating the
> __class__ of a module object feels dirty, but at least you get normal
> behavior w.r.t. properties.
Okay. Obviously I disagree, I think it's reasonable. But I'll assume
> Why is there no mechanism to add new descriptors that can work in this
I've updated the prototype to add one. I added it as
"collections.abc.InstanceDescriptor"; that's a base class you can
inherit from, and then your descriptor will work in a module.
Bikeshedding the name is fine.
> BTW. The interaction with import is interesting… Module properties
> only work as naive users expect when accessing them as attributes of
> the module object, in particular importing the name using “from module
> import prop” would only call the property getter once and that may not
> be the intended behavior.
I spent a fair amount of time thinking about this. The short answer is:
we /could/ fix it. We /could/ make it so that "from x import y", when
x.y is an instance descriptor, ensures that y honors the descriptor
protocol when referenced. We'd have to do it for three contexts:
* global scope (aka module scope)
* class scope
* function scope
The first two are pretty similar; create a proxy object that retains the
module instance so it remains "bound" to that. The third one is
trickier; it'd mean a new bytecode (LOAD_FAST_DESCRIPTOR), but it
wouldn't slow down people who didn't use it.
Anyway, long story short, I think this would be worse than simply having
"from x import y" only calling the getter once. As the Zen says:
special cases aren't special enough to break the rules.
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